A polarizing plate having a function of transmitting/shielding light is a basic constitutional element of a display device such as a liquid crystal display (LCD) together with a liquid crystal having a function of switching light. The application of LCD has been diversified from the earliest field of small devices such as calculators and clocks into laptop computers, word processors, liquid crystal projectors, liquid crystal TVs, car navigations and indoor/outdoor measurement instruments. Also application thereof to lenses having a polarization function and application thereof to sunglasses improved in visibility can be made. Recently, application to e.g., polarized glasses for use in compatible 3D TVs has been made. Since polarizing plates are used in a wide variety of fields as mentioned above, use condition thereof is diversified from low temperature to high temperature, low humidity to high humidity and low light intensity to high light intensity. For the reason, development of polarizing plates having high polarization performance and excellent durability has been desired.
At present, a polarizing film is manufactured by staining a polarizing-film base such as a stretched and oriented film formed of a polyvinyl alcohol or a derivative thereof or a polyene film (which is prepared (by producing a polyene) through dehydrochlorination of a polyvinyl chloride film or through dehydration of a polyvinyl alcohol film, and giving orientation thereto) with an iodine or a dichroic dye serving as a polarizing element or by adding an iodine or a dichroic dye to the film. Of these polarizing films, an iodine polarizing film (using an iodine as a polarizing element) is excellent in polarization performance but weak to water and heat. Thus, the iodine polarizing film has a problem in durability when the film is used for a long period of time in high temperature and high humidity conditions. To improve durability, various methods have been considered such as a method of treating the film with formalin or an aqueous solution containing boric acid and a method of applying a polymer film having low moisture permeability as a protecting film; however, these methods do not produce sufficient effects. In contrast, the dye polarizing film using a dichroic dye as the polarizing element has excellent moisture resistance and heat resistance compared to the iodine polarizing film but, in general, it fails to have sufficient polarization performance.
To a polymer film, several dichroic dyes are allowed to adsorb and orientation is given to prepare a neutral-color polarizing film. When two neutral-color polarizing films are stacked such that the individual orientation directions intersect at right angles (orthogonal arrangement) and then attached to a liquid crystal panel, if light leakage (color leakage) occurs in a specific wavelength within the visible light wavelength region, the color phase of the liquid crystal display sometimes changes in a dark state. Then, in order to prevent discoloration of a liquid crystal display due to color leakage in a specific wavelength in a dark state when the polarizing film is attached to a liquid crystal display device, in the neutral color polarizing film (which is prepared by allowing a polymer film to adsorb several dichroic dyes and giving orientation), it is necessary to uniformly decrease the transmittance of light in the orthogonal arrangement (orthogonal transmittance) within a visible light wavelength region.
Until recent years, to improve clearness of images in liquid crystal displays, images having a high brightness have been displayed. In hybrid cars and mobile terminals having such displays integrated therein, extension of a battery life has been desired. To satisfy the desire, reduction of power consumption is considered by liquid crystal display makers. In the circumstances, development of a neutral gray polarizing plate having good polarization performance and capable of increasing clearness of an image even in a lower brightness level has been desired. In the meantime, in in-car liquid crystal displays, since they are used in high-temperature and high humidity environments during summer (in car), development of a polarizing plate whose degree of polarization will not vary is desired. In past days, iodine polarizing plates having good polarization performance and displaying neutral gray were used. However, the iodine polarizing plates, as mentioned above, have a problem in that light resistance, heat resistance and moisture/heat resistance are insufficient due to use of iodine as a polarizer. To overcome this problem, a neutral gray polarizing plate using a dichromatic dye as a polarizer has come to be used; however, the neutral gray polarizing plate usually employs pigments of the three primary colors in combination, for uniformly improving transmittance and polarization performance in the whole visible light wavelength region. Then, dichroism pigments having satisfactory polarization performance had to be developed individually for three primary colors.
However, the bright line of a light source for a liquid crystal display varies depending upon the maker. Then, in developing a dichroism pigment having satisfactory polarization performance, it is important to design the wavelength of a pigment especially in accordance with the wavelength of the bright line. Because of this, development of three primary colors having excellent polarization performance in a limited wavelength region must be required.
As the dyes to be used in manufacturing the dye polarizing film, for example, water-soluble azo compounds described in Patent Literature 1 to Patent Literature 5 are mentioned.